Conventionally, various piezoelectric power generators have been proposed which generate electric power using a piezoelectric effect. Patent Document 1 discloses a piezoelectric power generator having a cantilever structure such as that illustrated in FIG. 12. This piezoelectric power generator includes a generating device 52 and a weight 53. One end of the generating device 52 is secured to a frame-like supporting member 51, and the other end of the generating device 52 is a free end. The weight 53 is provided for excitation and is attached to the free end of the generating device 52. The generating device 52 has a unimorph structure in which a piezoelectric element 52b is bonded to one principal surface of a metal plate 52a. The entire generating device 52 is in the shape of a rectangular parallelepiped. When an external vibration acts on the piezoelectric power generator, a free vibration is excited in the generating device 52 by the action of the weight 53, and an electric charge can be generated by a piezoelectric effect of the piezoelectric element 52b. The generated electric charge is extracted from charge collecting electrodes formed on the front and back surfaces of the piezoelectric element 52b. 
For example, like power generators that use vibration of human walk or vibration of bicycles or cars, there are power generators used in a relatively low-frequency vibration region. By bringing the natural frequency of a generating device close to the frequency of such a low-frequency external vibration, it is possible to significantly vibrate the generating device and increase the amount of power generation. The amount of power generation of a piezoelectric element is determined by a value obtained by dividing the product of the square of a piezoelectric constant of a piezoelectric body forming the piezoelectric element, the square of stress applied to the piezoelectric body, and a volume of the piezoelectric body by a dielectric constant of the piezoelectric body. If the material of the piezoelectric body is the same, the amount of power generation W is proportional to the product of the square of stress σ applied to the piezoelectric body and a volume V of the piezoelectric body as given by the expression W∝σ2×V.
Therefore, to increase the amount of power generation W, it is effective to increase both the stress σ applied to the piezoelectric body and the volume V of the piezoelectric body. In particular, the stress σ has a greater influence on the amount of power generation W than the volume V does. To increase the stress σ applied to the piezoelectric body, it is effective to increase the amount of displacement of the generating device. However, the amount of displacement of the generating device cannot be significantly increased simply by supporting the generating device 52 with the supporting member 51 in a cantilever manner.
Patent Document 2 discloses a piezoelectric power generator in which a plurality of generating devices having different resonance frequencies are supported in a cantilever manner. As illustrated in FIG. 13, this piezoelectric power generator includes a plurality of generating devices 61 to 63 each having one end secured to a base 60 and the other end being a free end, and weights 64 to 66 provided for excitation and attached to the respective free ends of the generating devices 61 to 63. The generating devices 61 to 63 have different lengths. This piezoelectric power generator is designed to generate electric power in a wide frequency range even when the frequency of external vibration varies. However, it is not possible to increase the amount of power generation at a specific frequency.
Patent Document 3 discloses another piezoelectric power generator. As illustrated in FIG. 14, this piezoelectric power generator includes a lever 71 having a cantilever structure in which one end thereof is secured to a metal angle 70 and the other end thereof is a free end, a weight 72 attached to the free end of the lever 71, and a multilayer piezoelectric element 73 connected to a midpoint (point of action) of the lever 71. This piezoelectric power generator amplifies an external force using the principle of leverage to increase the force applied to the multilayer piezoelectric element 73. However, since only a compressive load in the thickness direction acts on the multilayer piezoelectric element 73, the amount of distortion of the piezoelectric element is not large enough to significantly increase the amount of power generation.
Patent Document 1: Japanese Patent No. 3170965
Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-245970
Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-146663